The present invention relates to the field of photography and, more particularly, to a cassette for releasably holding a film unit and for locating the film unit at an exposure position therein in preparation for exposure.
The cassette embodying the present invention is particularly well suited for use with a large format, e.g. 8.times.10 or larger, film unit that forms a separate "negative" component of a self-developing film assemblage which also includes a "positive" component comprising an image receiving sheet having a rupturable container of fluid processing composition at one end thereof.
In the illustrated embodiment, the cassette is designed for X-ray applications and includes an interior bearing surface against which one side of the film unit is to bear in intimate contact, and an intensifying screen that is adapted to be urged into intimate contact with the opposite photosensitive side of the film unit to enhance image formation when the intensifying screen emits visible light in response to impingement of X-ray radiation thereupon. Provisions are also made for daylight loading of the film unit which is initially provided with an opaque envelope covering the photosensitive portion thereof for protection from ambient light until the film unit is safely located within the light excluding environment of the cassette housing.
Cassettes of this type generally include some type of mechanism for selectively applying pressure to the film unit in a direction transverse to the exposure plane to urge it into good contact with the intensifying screen. After the film unit is initially loaded into the cassette, it is desirable that this pressure be at a minimum so that the opaque envelope may be easily withdrawn through a light seal passageway at one end of the cassette. However, before the cassette is mounted on an appropriate apparatus for film exposure, increased pressure is applied to the film units so that one side thereof is urged into intimate contact with the film bearing surface while the intensifying screen is urged into intimate contact with the opposite side thereof.
Following exposure, it is desirable to once again minimize this pressure to facilitate the advancement of the exposed film unit through a light sealed passageway at the opposite end of the cassette so that it may be advanced into a processor in superposition with the image receiving sheet. Within the processor the superposed sheets are advanced between a pair of pressure applying rollers for releasing the fluid from the rupturable container and distributing it between the negative and positive sheets to initiate a well-known development and diffusion transfer process.
Commonly-assigned U.S. Pat. Nos. 4,200,384 and 4,156,568; and copending application U.S. Ser. No. 108,453, filed Dec. 31, 1979 and now U.S. Pat. No. 4,249,818 issued Feb. 10, 1981, disclose a variety of cassette receiving trays that are mountable on a commercially available large format film processor of the type described in commonly-assigned U.S. Pat. No. 4,019,194 and marketed by Polaroid Corporation, Cambridge, Mass.
This type of tray holds a positive sheet in alignment with the processor entry way. As the cassette is slidably inserted into the tray over the positive sheet, a mechanism therein guides a leader tab on the negative through a receiving slot on the positive leader to couple the sheets together for advancement between the rollers. The trays disclosed in the last two-mentioned tray patents are configured for use with X-ray cassettes (of the type more fully described in commonly-assigned U.S. Pat. No. 4,186,308 and copending application U.S. Ser. No. 108,457 filed on Dec. 31, 1979 and now U.S. Pat. No. 4,258,263 issued Mar. 24, 1981) and include mechanisms for automatically releasing a pressure plate assembly mounting the intensifying screen in response to locating the cassette at a fully-inserted position on the tray.
Another type of large format processing system is shown in commonly-assigned U.S. Pat. No. 3,364,835 wherein the cassette (described in more detail in commonly-assigned U.S. Pat. Nos. 3,371,208; and 3,383,507) initially holds both the positive and negative sheet of a self-developing X-ray film unit. Upon introducing the cassette into a processor, latch buttons on the side of the cassette are automatically actuated to release the latched cover section and relieve the pressure applied to the film unit by the intensifying screen thereby facilitating film withdrawal.
In order to maximize the convenience of using such a cassette and also simplify film processing operations, it should have certain characteristics. These include ease of loading a film unit into the cassette and removing the opaque envelope; the ability to easily move the intensifying screen into the pressure applying position and insure that it is reliably latched thereat with minimal manipulation by the operator; and compatibility with one or more of the above-mentioned trays to trigger the automatic release of the pressure urging the intensifying screen into contact with the film to facilitate removal. Also, the pressure latching system should be simple in its construction in the interest of reliability of operation and low manufacturing costs.
While the above-noted cassettes perform their intended functions satisfactorily, their mechanisms for releasably latching a pressure plate assembly mounting the intensifying screen in the lowered pressure applying position tend to be rather complex mechanically thereby resulting in relatively high manufacturing and assembly costs.
For example, the cassette disclosed in the previously-noted U.S. Pat. Nos. 3,371,208; and 3,383,507 include a pair of latching assemblies mounted in hollow side wall portions of the base section for sliding movement between latching and unlatching positions. The latching assemblies are normally spring biased into their latching positions. As the cassette cover section, which serves as a pressure plate for urging the intensifying screen into intimate contact with the film unit, is pivoted downwardly toward its closed position, latch tabs thereon engage angled cam surfaces on the latching assemblies and drive them rearwardly toward the unlatching position. When the latching assemblies are fully retracted, the latch tabs clear the bottom edge of the cam surfaces and the latching assemblies move forward over the tabs to their latching position.
Because the latching assemblies are initially located in the latching position, it is necessary to provide the camming structure to initially retract or cock the latching assemblies in preparation for their latching operation. The provision of the angled cam surface is an additional expense which does not contribute to the latching or unlatching function but rather is required to retract the latch assemblies in preparation for the latching operation. For any camming device to work properly, it must be carefully designed and then precisely located in the cassette with respect to the latch tabs to insure proper and reliable operation. Such required precision, of course, adds to the cost of manufacture. Also, because the latch assemblies engage the tabs deep within a recess formed in the side wall assemblies, the operator does not have a clear view of the latching operation and must depend on an audible click to assure him that the latching assemblies have indeed moved forwardly to the latching position.
The cassette disclosed in the previously-noted U.S. Pat. No. 4,186,308 utilizes movable cam members which interact with camming surfaces in the base section for moving the intensifying screen between its raised film release position and its lowered pressure applying position. Again, there is a high degree precision required for this mechanism to operate properly and therefore it is relatively costly to manufacture and assemble.
Likewise, the cassette described in copending application U.S. Ser. No. 108,457 utilizes six individual spring-biased latching members and corresponding camming structures for selectively effecting movement of the latching members between the latching and unlatching positions.
Therefore, it is an object of the present invention to provide a cassette of the general type described which is easy to use and simplified in its construction in the interest of reducing manufacturing costs.
It is another object to provide such a cassette that includes a latching system for releasably latching the pressure plate assembly in the pressure applying position which does not require expensive camming surfaces in its construction.
It is yet another object to provide such a cassette wherein the latching elements are visible to the operator so that he may easily ascertain that the pressure plate is securely latched.
Another object is to provide such a cassette which is compatible with a processing system wherein the pressure plate assembly latches are automatically released in response to inserting the cassette into a cassette receiving tray which also supports a positive image receiving sheet of a self-developing film assemblage.
Other objects of the invention will, in part, be obvious and will, in part, appear hereinafter.